The invention relates to a method for operating an internal combustion engine, especially of a motor vehicle, wherein fuel is injected into a combustion chamber in a rich operating mode and in a lean operating mode. In the method, a switchover takes place between the rich and the lean operating modes and nitrogen oxides are stored in a catalytic converter. The invention likewise relates to a corresponding internal combustion engine as well as to a control apparatus for an engine of this kind.
A method of this kind, an internal combustion engine of this kind and a control apparatus of this kind are, for example, known from a so-called gasoline direct injection. There, fuel is injected into the combustion chamber of the engine in a rich operating mode, for example, in a homogeneous operation during the induction phase or in a lean operating mode, for example, in a stratified charge operation during the compression phase. The homogeneous operation is preferably provided for the full-load operation of the engine while the stratified operation is suitable for idle operation and part-load operation. A switchover takes place between the above-mentioned operating modes in such a direct injecting engine, for example, in dependence upon a wanted desired operating mode.
Especially in the lean stratified operation, NOx components (that is, nitrogen oxides) are present in the exhaust gas which cannot be after-treated by a three-way catalytic converter. For this purpose, a storage catalytic converter is provided which stores nitrogen oxides in order to release the same in a subsequent rich operating mode of the engine. The storage catalytic converter must be continuously charged and discharged because the storage capability thereof is limited. This can, for example, be controlled (open loop and/or closed loop) with the aid of a lambda sensor ahead of the catalytic converter so that the storage capability of the storage catalytic converter is modeled.
Defects can occur in the lambda probe and/or in the modeling. These defects can lead to a permanently defective control (open loop and/or closed loop) of the charging and discharging of the storage catalytic converter.
It is the object of the invention to provide a method for operating an internal combustion engine with which the charging and discharging of the catalytic converter is correctly controlled (open loop and/or closed loop) also over a long time span.
This object is solved in a method of the above-mentioned type in accordance with the invention in that the lambda of the exhaust gas is measured after the catalytic converter and that a first time span is measured after a switchover of the engine into the lean operating mode with this time span elapsing until the measured lambda is lean. In an internal combustion engine and a control apparatus of the above-mentioned type, the object is solved in correspondence to the invention.
Accordingly, a second lambda probe is arranged downstream of the catalytic converter and the lambda of the exhaust gas is measured therewith. If the control apparatus switches the engine into the lean operating mode, then that time span is measured thereafter which elapses until the lambda, which is measured by the second lambda sensor, becomes lean. This time span is a quantity which characterizes the operating state of the storage catalytic converter.
A value can be determined and pregiven, for example, by the manufacturer of the catalytic converter, which defines the limit of a correct operating state of the storage catalytic converter. The measured time span can then be compared by the control apparatus to this limit. If the limit is not exceeded, then the catalytic converter operates in a correct manner. However, if the limit is exceeded, this means that the storage catalytic converter is or works no longer in a permissible operating state. In this case, the control of the engine (open loop and/or closed loop) can be so influenced by the control apparatus that a correct operation of the storage catalytic converter is again attained.
In this way, it is possible to ensure the correct charging and discharging of the storage catalytic converter also for the long term with the aid of the second lambda sensor and a corresponding operation of the engine.
In an advantageous further embodiment of the invention, after a switchover of the engine into the lean operating mode, a second time span is measured in which the engine is operated lean. The first time span and the second time span are then coupled. Preferably, a value is determined which defines the ratio of the first time span to a third time span. The sum of the first time span and the third time span yields the second time span. In this way, it is possible in an especially simple but precise manner to obtain a reliable statement as to the operating state of the storage catalytic converter.
In a further advantageous embodiment of the invention, the control (open loop and/or closed loop) of the engine is influenced if the first time span or a value dependent thereon exceeds a pregiven value. The pregiven value characterizes the maximum permissible operating state of the storage catalytic converter which may not be exceeded.
It is especially advantageous when the time span, in which the engine is driven rich, is extended and/or when the time span wherein the engine is driven lean is shortened. This defines reactions of the control apparatus to the maximum permissible operating state of the storage catalytic converter being exceeded.
As a further advantage, it is possible that the influencing of the engine intervenes adaptively in the control (open loop and/or closed loop). In this way, the model of charging and discharging the storage catalytic converter can also be adapted to long-term changes thereof.
Of special significance is the realization of the method of the invention in the form of a control element which is provided for a control apparatus of an engine, especially of a motor vehicle. A program is stored on the control element which is capable of being run on a computer, especially on a microprocessor, and this program is suitable for executing the method according to the invention. In this case, the invention is realized by a program stored on the control element so that this control element, which is provided with the program, defines the invention in the same way as the method which the program can carry out. Especially an electric storage medium can be used as a control element, for example, a read-only-memory or a flash memory.
Further features, application possibilities and advantages of the invention will become apparent from the following description of embodiments of the invention which are illustrated in the drawing. All described or illustrated features define the subject matter of the invention by themselves or in any desired combination independently of their summary in the patent claims or their dependency as well as independently of their formulation or presentation in the description and/or in the drawing.